The nuclear industry has designed power stations that are capable of producing large amounts of energy from a small amount of nuclear fuel. It has thus developed steam generators requiring advanced heat transfer systems, which can be characterized as pressurized water systems or, for breeder reactors or fast neutron reactors, systems based on molten sodium metal.
The liquid sodium used as heat transfer fluid in the primary and secondary circuits of fast neutron reactors has to be treated when the circuits are dismantled. To reduce the chemical risk of storing the sodium in its liquid metallic form, it is converted to concentrated sodium hydroxide solution.
The approach adopted hitherto consists of a two-step conversion of the liquid sodium metal potentially contaminated by radioactive isotopes: a hydrolysis step to convert said sodium to sodium hydroxide, and a solidification/stabilization step to convert the sodium hydroxide to a solid whose stability is compatible with storage at an appropriate central point.
The process according to the present invention can be applied to this second step.
It is also desirable to be able to solidify and stabilize concentrated aqueous sodium hydroxide solution contaminated by other pollutants. Possible examples of such pollutants which may be mentioned are organic products and heavy metals (zinc, lead, arsenic, etc.).
The problem therefore consists in incorporating aqueous sodium hydroxide solution into a solid matrix with a high loading rate of the aqueous solution.
Patent FR 2804103 has already disclosed a process for the conditioning of aqueous sodium hydroxide solution to give solid compounds of the “nepheline” type. These are obtained by reacting the sodium hydroxide with compounds that provide silica and alumina, such as metakaolin, bentonite, dickite, halloysite and pyrophillite. After a primary reaction, which takes place at ambient temperature to form a precipitate of the zeolite type (cancrinite), a second treatment phase at a temperature between 1000 and 1500° C. makes it possible to convert this zeolite to very sparingly soluble nepheline (sodium aluminosilicate).
It is clearly understood that such a process is very efficient since it enables all the sodium present to be converted to a practically insoluble mineral. However, this process has the disadvantage of requiring two treatment steps, the second of which consists of a high-temperature heating phase.
The main object of the invention is to provide a process in which concentrated aqueous sodium hydroxide solution can be solidified directly at ambient temperature.